A new interpretation of plasmodesmatal ultrastructure

Area of Science:

• Plant cell biology
• Plant anatomy
• Cellular ultrastructure

Background:

• Plasmodesmata are crucial for intercellular communication in plants.
• The ultrastructure of plasmodesmata, particularly in specialized cells like xylem ray cells, requires detailed investigation.
• Previous studies have proposed various structures traversing plasmodesmata, including endoplasmic reticulum continuities.

Purpose of the Study:

• To elucidate the detailed ultrastructure of plasmodesmata in young willow xylem ray cells.
• To determine the nature of the central tubule (desmotubule) within these plasmodesmata.
• To clarify the relationship between plasmodesmata, endoplasmic reticulum, and cytoplasmic microtubules.

Main Methods:

• Transmission electron microscopy was used to examine the ultrastructure of plasmodesmata.
• Detailed measurements of plasmodesmatal components, including the desmotubule and surrounding gap, were performed.
• Comparison of desmotubule dimensions with cytoplasmic microtubules and potential endoplasmic reticulum structures.

Main Results:

• Plasmodesmata in willow xylem ray cells lack direct intercellular cytoplasmic continuity.
• Each plasmodesma contains a desmotubule (200 Å diameter) with a distinct subunit structure and a central rod.
• The desmotubule is appressed to the plasmalemma at the cell ends, and a gap exists centrally, precluding ER passage.

Conclusions:

• Desmotubules are distinct from cytoplasmic microtubules and may originate from nuclear spindle fibers.
• The endoplasmic reticulum does not appear to traverse these plasmodesmata.
• The plasmalemma lining the plasmodesmatal canal exhibits particulate subunits, potentially for membrane stability.